Slides and talking points are provided courtesy of AAPG Visiting Geoscientist Fred W. Schroeder.

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The Basics of Prospecting

• Printing Instructions:
  • “A Quick-Look Evaluation”
    • one document, 3 pages, letter size, color (may be OK in B&W)
• Supplies:
  • Pen or a pencil (for taking notes); Colored pencils: red, yellow, blue, green and #2 (graphite); Eraser

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Slide 1

• Introductory slide with some ‘eye candy’
• 4 vibrator trucks collecting land seismic data
• Offshore drilling platform
• View of seismic + a horizon and some faults (upper right)
• Seismic horizon color-coded by 2-way time cut by a fault (lower left)
• Two people working data on paper

 

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Slide 2

• In exploration there are 4 questions – as listed
• HC = hydrocarbon
• Risk = (1 – chance of success) or chance of success = (1 – Risk); e.g. a proposed well may have a 75% chance of success, which can also be stated as a 25% risk – 1 out of 4 chance of failure

 

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Slide 3

• Rube Goldberg drew complicated contraptions to do simple tasks – like the board game Mouse Trap
• First thing you need is a kitchen...
• Then you need a container...
• Usually the kitchen and container are not connected, so you need plumbing...
• Next you need to place the well in the right location
• If you miss the container – a dry hole and no money
• But if everything ‘works’ – you get oil out of the ground and money in the bank
• Being a bit more technical, we need
• Source – rock rich in organic carbon that has the right temperature & pressure conditions so that the kinetics transform organic matter into oil & gas molecules
• Reservoir rock with a trapping geometry capped by a sealing rock
• Migration pathways that allow the oil & gas molecules to move from the source to the trap on a geologic time scale

 

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Slide 4

• The kitchen is where organic matter dispersed within a source interval has undergone the temperature/pressure history necessary for oil and gas to be generated and expelled
• Source intervals are organic rich. They are rated based on
• Their organic carbon content (TOC = total organic carbon) and
• Their richness (HI = hydrogen index which controls oil vs. gas)
• Most source intervals are shales
• The best source rocks were deposited under reducing conditions with TOC over 12% carbon by weight (can be as high as 18%)

 

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Slide 5

• A reservoir is a rock with enough porosity (pore space) and permeability (connectiveness) that we can produce (extract) oil and gas out of it
• Most reservoirs are in clastic units of sand-size or larger particles (sandstones, conglomerates) or in coarse carbonates (e.g., reefs)
• A trap is a 3D configuration in the subsurface that allows oil/gas to pool in significant quantities
• Traps resulting from faults or other structural features are called structural traps – they are the easiest to recognize
• Traps resulting from the wedging out of a reservoir-quality rock, either due to depositional thinning or post-depositional erosion, are called stratigraphic traps
• Seals are rock layers that prevent leakage of HCs from the trap
• The most common seals are shales and evaporites
• Top seal prevents leakage up through the top of a reservoir
• To have a trap, we also need lateral seals so that HCs don’t leak out of the sides of a trap (usually more critical with stratigraphic traps).

 

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Slide 6

• HC migration is the process of moving droplets of oil and gas from the source to the reservoir
• Primary migration is getting the HC out of the source interval
• Secondary migration is moving the HC in carrier beds and up faults/fractures to the reservoir
• Migration parallel to the depositional units occurs in sand and silt beds that serve as carrier beds
• Migration from one stratigraphic level to another is called cross-stratal migration
• It commonly occurs via faults and fractures
• Most cross-stratal migration is in an upward direction (buoyant forces) but depending on pressure gradients HCs can move down into carrier beds if the pressure gradient is downward.

 

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Slide 7

• Here is a cross-section through a sedimentary basin
• The ‘granite’ pattern represents non-sedimentary (basement) rocks
• There are ~12 major depositional unit (layers)
• Someone, a basin modeler, has predicted the depths at which:
• Mostly oil would be generated – the ‘oil window’
• Only gas would be generated – the ‘gas window’
• Where no more HC would be generated – below the gas window
• Now all we have to do is:
• Figure out where the source rocks are.
• Identify potential reservoir units
• Locate potential traps that are capped by a sealing lithology
• And hypothesize HC migration pathways
• Then we can predict where there are oil and gas fields just waiting to be discovered – simple!
• If we are working a basin in which fields have been discovered, we can ‘reverse engineer’ the HC system
• For example, if we know oil is in the shallow reservoir on the right, we know HC migrated into it somehow
• As shown by the blue arrow, we might call upon HC migration up the fault
• This would connect our ‘kitchen’ to our known field
• Of course we have to consider the 3D basin geometry – not a single cross-section

 

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Slide 8

• There are other elements to consider for the HC System
• Timing – did the trap exist when HC migration occurred
• Obviously if HC migration occurred before the trap existed, the trap will be empty or severely under-filled
• Fill & Spill – if the trap volume is small compared to the volume of generated HC, then the trap has been overfilled and excess HC has spilled
• Since free gas displaces oil, an overfilled trap may hold gas while spilling oil
• The spilled oil could be trapped further up the overall migration path
• We’ll see a cartoon example of this on the next slide
• Preservation – if oil is trapped, there are conditions that can degrade the oil with time
• If the reservoir gets too hot, the oil can be cooked (cracked) to gas
• If the reservoir is shallow and cool, bacteria can feed off the oil and spoil (degrade) it

 

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Slide 9

• This slide illustrates some basic concepts about HC fill and spill
• Trap A is closest to the kitchen and will fill first
• It has a synclinal spill point on the right
• HCs spilled from Trap A will migrate up to Trap B
• Trap B has a fault leak point
• Early charge from an oil-prone source consists of oil with a minor amount of gas
• Trap A starts to fill with oil and dissolved gas
• As time passes, significant oil with a large proportion of gas reaches Trap A
• If there is more gas than can be dissolved in the oil, then a free gas cap forms
• The gas cap will displace oil, so only oil (with some dissolved gas) will spill
• Eventually the source will become over-mature, only generating gas
• If enough gas reaches Trap A, it will become entirely filled with gas – all the oil being displaced (spilled)
• Trap B now has a free gas cap and an oil leg, with oil spilling out of Trap B at the fault leak point
• Perhaps there is a Trap C further up the migration path where oil spilled from Trap B is collecting
• A good explorationist would start to search for more traps up the migration pathway